The present invention relates to a sensing unit mountable on a wall of a duct for detecting a plurality of parameters of a fluid in the duct.
Temperature and humidity are primary factors in the comfort and quality of an indoor environment. While temperature is important to comfort, the humidity is a substantial factor in determining whether a specific temperature is comfortable. Temperature is commonly regulated as a function of the relative humidity in a space and humidifiers, to control the relative humidity, are often a part of the heating, cooling and ventilation systems of office buildings and industrial plants.
Relative humidity may be sensed by a sensor that comprises a polymer that is typically mounted on a porous ceramic plate and has a resistivity that changes as a function of the humidity. This type sensor is usually not sufficiently accurate for use with a ventilation system and is subject to deterioration in harsh environments.
A second type of humidity sensor employs a capacitor in which the dielectric comprises environmental air. Since the dielectric constant of air is one and the dielectric constant of water is approximately 80, changes in the relative humidity changes the dielectric constant of the air separating the capacitor plates, and, hence, the capacitance of the sensor. Variation in capacitance can be used in a number of ways in circuits to provide an electrical output that is indicative of the relative humidity.
The accuracy of the variable capacitance sensor depends upon the accuracy of the sensor's nominal capacitance which can be altered by the way in which the capacitor is shipped, handled or otherwise introduced to the environment. Cota, U.S. Pat. No. 5,844,138, discloses a humidity sensing device that includes a humidity sensitive capacitor comprising part of an oscillator circuit. The frequency of the oscillator is a function of the capacitance of the humidity sensitive capacitor which, in turn, is a function of the relative humidity. The true capacitance of the humidity sensitive capacitor is measured against a known standard and stored in a memory in the humidity sensing device. A microprocessor uses the true capacitance data stored in the memory to correct the relative humidity measurements made with the device to account changes in capacitance resulting from aging or from shipping and handling of the device. A voltage divider network in the humidity sensing device provides temperature compensation for the relative humidity measurements. Cota also discloses an apparatus for supporting the humidity sensor in a stream of fluid flowing in a duct. An enclosure with an attached sleeve is bolted to the exterior of the duct with the sleeve projecting through a hole in the duct's wall. The humidity sensitive capacitor is secured in the end of a tube which passes through the sleeve. A swage nut compresses the sleeve to secure the tube and the humidity sensitive element in the fluid flowing in the duct.
Temperature is commonly measured with a thermistor or a resistance temperature detector (RTD) which exploit the predictable change in electrical resistance of certain materials when they are exposed to changing temperatures. Thermistors and RTDs can be very compact enabling a temperature sensor to be included with the humidity sensor in a mounting similar to that disclosed by Cota.
Carbon dioxide (CO2) is a product of human respiration and, while high levels of carbon dioxide are toxic to humans, the concentration of carbon dioxide in an indoor environment is commonly used as a surrogate to indicate the presence of other indoor pollutants that may cause occupants to grow drowsy, have headaches, or function at a lower activity level. Since human respiration is a primary source of carbon dioxide in indoor environments, building codes typically specify the amount of outdoor air to be added to an interior space by the ventilation system on the basis of the occupancy of the space. In the past, ventilation systems commonly maintained a ventilation rate, at all times, that was sufficient for full occupancy of the space. However, heating, cooling, humidifying and moving this volume of air at times when the occupancy is low is wasteful of energy and expensive. Demand controlled ventilation seeks to vary the amount of outside air added to a space, under conditions of variable and intermittent occupancy, to optimize the comfort and well being of occupants and reduce energy consumption. Carbon dioxide concentration is used as an indicator of the occupancy and as a control parameter for demand controlled ventilation.
The presence of carbon dioxide is typically detected with either a chemical sensor or a non-dispersive infrared sensor. Chemical sensors comprise materials that are sensitive to the presence of CO2 and while they typically consume little energy and can be miniaturized, they have a relatively short lifespan and are subject to drift effecting the short and long term accuracy of the sensor. Non-dispersive infrared sensors comprise a source and a detector of infrared light disposed at opposite ends of a light tube and an interference filter to prevent light, with exception of light absorbed by the gas molecules of interest, from reaching the detector. A gas to be tested is introduced to the light tube and the absorption of a characteristic wavelength of light is measured to determine the presence of CO2 in the gas. Non-dispersive infrared sensors can be expensive but are commonly used because no other known method works as reliably to detect CO2.
A CO2 sensor can be mounted on a wall in the space to be monitored in a manner similar to the installation of a thermostat. The location of the sensor should be selected to expose the sensor to air that is indicative of general conditions within the occupied zone. Locations near doors, windows and air vents or close to where people would regularly sit or stand should be avoided because the CO2 may be locally diluted by air from outside or concentrated by the local activity. A large number of wall mounted sensors are typically required because each sensor is only exposed to the local environment and at least one sensor is typically required in each space. Sensors for humidity and temperature may be combined with a wall mounted CO2 sensor to reduce the number of sensor installations.
CO2 sensors may also be installed on the duct work of an air handling system to detect the concentration of CO2 in the air flowing in the ducts. Duct mounted CO2 sensors are typically arranged to detect CO2 in fluid that is a returning from a space but may also be mounted in the air intake for the ventilation system to measure the CO2 in the intake air. While a ventilation system comprising a plurality of zones typically incorporates a number of sensors, a duct mounted sensor can serve a plurality of zones reducing the required number of sensors. Duct mounting of the CO2 sensor is best applied where the ventilation system operates continuously and where the monitored return airstream serves one or more zones that have similar levels of activity and occupancy at similar times.
In addition to monitoring temperature, humidity and carbon dioxide, sensors may be installed in a duct for detecting other constituents of the fluid flowing in the duct, such as, carbon monoxide (CO), volatile organic compounds (VOC) and smoke.
In addition to monitoring parameters that effect air or fluid quality, ventilation and other fluid circulating systems commonly include sensors, for example pressure sensors, for monitoring the performance of the system. The pressure in a duct is typically monitored by a pressure sensing unit comprising a housing that is attachable to the exterior surface of a wall of the duct and includes a duct wall penetrating tube to communicate the fluid pressure from a point distal of the inside surface of the duct wall to a differential pressure transducer located in the housing. A differential pressure transducer typically comprises a transducer element, such as a diaphragm or a strain gauge, that is arranged to respond to a difference between the pressure applied at one input to the transducer element and the pressure applied at a second input and a measuring system to output a signal indicative of the pressure differential. For example, Crider, U.S. Pat. No. 6,122,972, discloses a pressure sensor comprising a dielectric material interposed between a first conductive surface and a dissimilarly shaped, second conductive surface that is isolated from the first conductive surface. The dielectric material is movable with respect to at least one of the conductive surfaces in response to a pressure difference at the inlet ports. A measurement system connected to at least one of the conductive surfaces measures the change in capacitance between the conductive surfaces as a result of movement of the dielectric material in response to a change in differential pressure. Duct mounted sensors may be arranged to sense the gauge pressure or the absolute pressure of the fluid in the duct or the pressure differential between the ends of a section of a duct.
Combining a plurality of sensors in a single enclosure can reduce the cost the sensing units required for a fluid circulating system, such as a ventilation system for a building. Moreover, if a plurality of sensors can be installed at a single insertion point in a duct, the cost of installing and maintaining the ventilation system and the likelihood of leakage can be reduced.
What is desired, therefore, is a sensing unit enabling installation of a plurality of sensors, including a pressure sensor, at single insertion point in a fluid circulation duct.